1. Field of the Invention
This invention relates to a thermal spraying composite material containing molybdenum boride, and more particularly, to a thermal spraying composite material for forming a coat to protect mechanical equipment from corrosion by any molten light metal or alloy, such as aluminum, zinc or alloys of these.
2. Description of Related Art
Die casting, gravity casting, or differential pressure casting have been usual processes for casting a product from a metal having a relatively low melting point, such as aluminum, zinc or magnesium.
Differential pressure casting is, among these, considered suitable for making large castings having fewer internal defects. FIG. 2 shows an apparatus employed for differential pressure casting. A suction port 6 is used to create a lower pressure in a mold 5 than in a holding furnace 1, so that a molten metal 10 may rise from the holding furnace 1 through a stoke 2 and form a laminar flow through a sleeve 4 to fill the mold 5 for one cycle of the casting operation. When the molten metal has solidified on the inner surface of the mold 5, the next cycle of the casting operation is started, and the remaining molten metal flows down into the holding furnace 1 through the sleeve 4.
The sleeve 4 has its inner surface washed by the molten metal 10 having a high temperature during each cycle of the casting operation, and thereby corroded, and is eventually fractured. The higher temperature the molten metal has, the shorter life the sleeve 4 has.
The molten light alloys have usually been used for casting at relatively low temperatures in the range of 700-750.degree. C. and the protective coats of the sleeve 4 and the mold, have been made of, for example, a mixture of tungsten carbide and cobalt having a cobalt content of 12% by weight, as described in the Japanese Unexamined Patent Application No. Hei 7-62516.
Many foundries have, however, come to employ higher molten metal temperatures in the range of 750-850.degree. C. for making products of higher accuracy by the differential pressure casting.
When exposed to any such higher molten metal temperature, the protective coats of tungsten carbide and cobalt have been found lacking in durability, and particularly in oxidation resistance, and heavily worn by oxidation not only on the sleeve 4, but on the inner surface of the mold 1 as well, owing to low oxidation resistance of tungsten carbide at high temperature. A greatly shortened mold life has led to increase in the cost of the casting operation.